1. Field of the Invention
This invention generally relates to a dimmer control circuit, and more particularly to a low visual noise dimmer control circuit by adjusting current beam density.
2. Description of Related Art
Liquid crystal displays (LCD) have been widely used to replace the conventional CRT displays. As the semiconductor manufacturing technology advances, LCD has several advantages such as low power consumption, light weight, high resolution, high color saturation, and longer lifetime, and can be used in state-of-the-art electronic devices such as digital cameras, notebook computers, desktop computers, mobile phones, personal digital assistant (PDA), global positioning system (GPS), etc.
Because LCD is not self-emitting, a cold cathode fluorescent lamp (CCFL) is used as a light source. For stable operation of the cold cathode fluorescent lamp, the power source is a sinusoidal signal having a frequency between 30 KHz and 80 KHz without DC component. The stable operational voltage is approximately a constant. The brightness of the lamp depends on the current through the lamp.
For a large size LCD application, the signal with a high frequency and a high voltage for driving the lamp will leak via the parasitic capacitor between the lamp and the panel. Hence, when the current through the lamp is small, the so-called thermal meter effect is generated in which the ground end is darker than the high-voltage end of the lamp, or the lamp cannot emit light. To overcome the thermal meter effect, the conventional method dims the lamp by fixing the amplitude of the current through the lamp and adjusting the current beam density to obtain a maximum dimming range.
FIG. 1 is the block diagram for the conventional jitterized pulse width modulation brightness control circuit. FIG. 2 is a schematic diagram showing the relationship between the brightness control pulse signal and the fluorescent light driving current signal of the circuit of FIG. 1. As shown in FIG. 1, the brightness control signal Con is sent to the inverter 110 to control the fluorescent light driving current signal Id. FIGS. 2(a), (b), and (c) illustrate the outputted wavelength of the fluorescent light driving current signal ID controlled by three different pulse widths. FIG. 2(a), (b), (c) show that the brightness is 100%, 20%, and 50% respectively.
To prevent users from visual interference due to the on/off frequency of the fluorescent light, the frequency of the brightness control signal Con has to be kept at a certain level, such as, 200 Hz. Hence, eyes of an individual will not blink due to the changes of the brightness of the fluorescent light.
Because the frequency of the brightness control signal is fixed based on the required brightness, when the lamp is used for LCD back light, the back light signal would interfere the vertical and horizontal video signals due to the frequency difference. The frequency difference between the back light signal and the video signals would cause the so-called “fan effect”, in which ripples are formed on the display. Further, the frequency of switching the inverter also affects the power source of the inverter, which causes the power source to generate the ripples having the same frequency as the brightness control signal. The generated ripples also affect the scan signal, which causes glistening on the display.
To avoid interference generated between the back light signal and the vertical and horizontal scan signals due to the frequency difference, one can synchronize and double the frequencies of the brightness control pulse signal and the horizontal scan signal. However, it requires a higher cost. Another solution is to increase the frequency of the brightness control signal to reduce the interference to the power source. However, for a large size LCD, it is more and more difficult to achieve a low-noise and a wide dimming range lamp solution.